The present invention relates to a wideband phase shifter, and, more particularly, a 180-degree digital bit in which the 180-degree phase shift is obtained independent of the electrical length of the transmission line sections which make up the phase shifter.
Phase shifters are devices in which a difference in the phase of an electromagnetic wave of a give frequency propagating through a transmission line can be shifted. Such phase shifters are utilized in many microwave systems and in particular are required for electronic beam steering in phased array radar systems. A typical prior art wide-band 180-degree phase shifter bit is constructed by placing switching diodes at the coupled and through ports of a branch line or Lange coupler. By switching the diodes off and on, the signal appearing at the output ports of the coupler exhibit a relative 180-degree phase shift. Such a phase shifter is limited to generally less than an octave bandwidth at microwave frequency because of phase shift deviations from the desired 180 degrees.
Other types of phase shifters are found in the prior art in which a change in phase is obtained by utilizing one of a number of lengths of transmission line to approximate the desired value of phase change. The various lengths of transmission line are inserted and removed by high speed electronic switching. Semiconductor diodes and ferrites are the devices commonly employed in digital phase shifters. One such digitally switched phase shifter is a parallel-line configuration in which the proper transmission line length is selected from among many available parallel lines. An alternative phase shifter is a seriesline or a cascaded multi-bit digitally switched phase shifter. Such phase shifters are described more generally in "Introduction to Radar Systems", 2nd Edition, by M. L. Skolnick, 1982, pg. 286.
Such prior art phase shifters have in general not been found to be usable over a wide bandwidth due to high insertion losses and a non-linear phase shift versus frequency characteristic throughout the band of interest.
It is desirable to provide a 180-degree phase shifter bit which is operable with a linear response over the range of 4.5 to 18 GHz. It is also desirable that the phase shifter be as small as possible and compatible with monolithic microwave integrated circuit manufacturing techniques.